title: Postnatal ablation of osteoblast Smad4 enhances proliferative responses to canonical Wnt signaling through interactions with β-catenin.
abstract: Canonical Wnt (cWnt) signaling through β-catenin regulates osteoblast proliferation and differentiation to enhance bone formation. We previously reported that osteogenic action of β-catenin is dependent on BMP signaling. Here, we further examined interactions between cWnt and BMP in bone. In osteoprogenitors stimulated with BMP2, β-catenin localizes to the nucleus, physically interacts with Smad4, and is recruited to DNA-binding transcription complexes containing Smad4, R-Smad1/5 and TCF4. Furthermore, Tcf/Lef-dependent transcription, Ccnd1 expression and proliferation all increase when Smad4, 1 or 5 levels are low, whereas TCF/Lef activities decrease when Smad4 expression is high. The ability of Smad4 to antagonize transcription of Ccnd1 is dependent on DNA-binding activity but Smad4-dependent transcription is not required. In mice, conditional deletion of Smad4 in osterix(+) cells increases mitosis of cells on trabecular bone surfaces as well as in primary osteoblast cultures from adult bone marrow and neonatal calvaria. By contrast, ablation of Smad4 delays differentiation and matrix mineralization by primary osteoblasts in response to Wnt3a, indicating that loss of Smad4 perturbs the balance between proliferation and differentiation in osteoprogenitors. We propose that Smad4 and Tcf/Lef transcription complexes compete for β-catenin, thus restraining cWnt-dependent proliferative signals while favoring the matrix synthesizing activity of osteoblasts.
authors.affiliation: Department of Internal Medicine, Division of Bone and Mineral Disease, Washington University School of Medicine, 660 South Euclid, Campus Box 8301, Saint Louis, MO 63110, USA.
authors.forename: Valerie S
authors.initials: VS
authors.lastname: Salazar
authors.forename: Nicholas
authors.initials: N
authors.lastname: Zarkadis
authors.forename: Lisa
authors.initials: L
authors.lastname: Huang
authors.forename: Marcus
authors.initials: M
authors.lastname: Watkins
authors.forename: Jacqueline
authors.initials: J
authors.lastname: Kading
authors.forename: Sheri
authors.initials: S
authors.lastname: Bonar
authors.forename: Jin
authors.initials: J
authors.lastname: Norris
authors.forename: Gabriel
authors.initials: G
authors.lastname: Mbalaviele
authors.forename: Roberto
authors.initials: R
authors.lastname: Civitelli
mesh_terms: D000818:Animals; D001665:Binding Sites; D055396:Bone Morphogenetic Protein 2 / Q000502:physiology; D002113:Calcification, Physiologic; D002460:Cell Line; D049109:Cell Proliferation; D019938:Cyclin D1 / Q000235:genetics / Q000378:metabolism; D005786:Gene Expression Regulation; D055786:Gene Knockout Techniques; D051379:Mice; D057507:Mice, 129 Strain; D008810:Mice, Inbred C57BL; D008938:Mitosis; D010006:Osteoblasts / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D051901:Smad4 Protein / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic; D060449:Wnt Signaling Pathway; D051176:beta Catenin / Q000378:metabolism*
chemical_list: C525719:Bmp2 protein, mouse; D055396:Bone Morphogenetic Protein 2; C495267:CTNNB1 protein, mouse; C530105:Ccnd1 protein, mouse; D051901:Smad4 Protein; C490810:Smad4 protein, mouse; D051176:beta Catenin; D019938:Cyclin D1
keywords: Osteoblast; Proliferation; Smad4; β-catenin